1. Field of the Invention
The present invention relates to coreless-type brushless direct-current("BLDC") motor and a method of producing stator assembly. More particularly, it relates to a double rotor/stator-type coreless BLDC motor and a method for making its stator assembly which doubles advantages of the conventional respective core and coreless DC motors and obviates their disadvantages by symmetrically providing a pair of disk-shaped rotors to upper and lower parts of a disk-shaped stator or by symmetrically mounting rotors between a pair of stators, thereby avoiding the rotor's axial vibration, creating high torque, and minimizing power consumption.
2. Description of the Related Art
Coreless-type BLDC motors may be classified into cylindrical core (radial) and coreless (axial) ones depending on whether a stator core exists.
The core-type BLDC motors are characterized as an internal magnet-type motor and an external magnet-type motor. The internal magnet-type motor includes a cylindrical coil-wound stator and a rotor of cylindrical permanent magnets provided to a plurality of protrusions formed on its inner circumference so as to be of electromagnetic construction. In the external magnet-type motor, a stator around which a coil is wound and a rotor having cylindrical permanent magnets are provided to a plurality of protrusions formed on its outer circumference.
Since its magnetic circuit has an axial-symmetric structure, the core BLDC motor makes little noise during operation and is suitable for low-speed rotation, creating desirable torque. This core BLDC motor, however, results in a waste of materials for making a stator and requires great expense for facility investment for mass production. In addition, since the core BLDC motor's stator and rotor are of complicated structure, it is not easy to make the motor compact, and it cannot assure high efficiency or the production of desirable amounts of torque.
A coreless BLDC motor was proposed in order to solve the above-described problems. Referring to a coreless BLDC motor depicted in FIG. 1, rotors 5 that each consists of an annular magnet 1 and a yoke 3 are fixed to a shaft 7, and stators 13 around which a plurality of bobbinless-rectangular stator coils 11 are wound are fixed to a casing 10. One end of the shaft 7 is rotatably joined to the casing 10 by means of a pair of bearings 15.
This coreless BLDC motor has a magnetic circuit axially created between the rotors 5 consisting of a set of N-and-S-pole magnets 5A and 5B (refer to FIG. 3) and the stators 13 about which a plurality of the stator coils 11 generating electromagnetic force are wound. Thus, even if a buffer spring 17 is inserted between a pair of the bearings 15, the coreless BLDC motor generates great axial vibrations due to the stators' attracting or repelling force and their unequal magnetization. Besides, the axial vibrations induces a resonance of the overall system employing the coreless BLDC motor during operation, thereby increasing the noise. Accordingly, the motor's efficiency is not decreased during high-speed rotation but gives rise to much noise.
In conclusion, the above-described coreless BLDC motor saves materials and has an advantageous yield aspect compared to the core BLDC motor. Moreover, it is possible to make it compact, which lowers the overall production costs and enhances its efficiency. The coreless BLDC motor, however, creates much noise due to axial vibrations during operation.